This invention is directed to dial-up modems which communicate over the public switched telephone network and more specifically to such a modem which permits direct connection with a dedicated Host line which uses bisynchronous 3270 communication protocol without requiring system operator assistance.
Older computer systems used a bisynchronous (bisync) communications protocol which assumed the use of dedicated lines and compatible bisync terminals. With the current availability of relatively low cost modems, it is desirable to be able to communicate over the public switched telephone network with such computers. Control units (CU), such as an IBM 3704/3705 or NCR Comten, are used to handle communications for these computer systems. As used herein, Host refers to such CU's and not the mainframe computer itself.
Because the Host periodically interrogates or polls the terminal expected to be active on a dedicated communication line and expects certain acknowledgements, a conventional modem connected to such a line must always be connected with a bisync terminal or terminal emulator in order to satisfy the required protocol. Thus requires the use of a dedicated line between the modem at the Host and the modem at the terminal, and constant communication with the terminal so that it can respond to the polls.
To establish communication between the modem at the Host and a modem at a terminal emulator over a dial-up telephone line, a system operator at the Host site had to be alerted by the remote user and cause the Host to reactivate communications on the dedicated line connected with the modem. The reactivation was needed since the Host would have previously found the expected terminal to be unavailable due to no response to polls and would have inactivated communications. The need for system operator assistance in order to establish communications over a dial-up telephone line represents a substantial impediment.
One approach to this problem is to use a special purpose microprocessor controlled device as an interface between the Host and the modem, such as a LineMaster made by Microframe, Inc. Its purpose is to satisfy the Host if a remote terminal is not active and to be transparent when a remote terminal is active. Although this approach does allow dial-up telephone line communications, it represents: a substantial cost as compared to the cost of the modem, an increased possiblilty of a communication link failure, and an inability to take advantage of controls available internally in the modem.